High power solid-state optical amplification process and system

ABSTRACT

A high power solid-state non-regenerative optical amplification system ( 100 ) for amplifying a pulsed optical beam, includes a first optical amplification crystal (C 1 ) and a second optical amplification crystal (C 2 ) for amplifying the optical beam; optical pumping elements for longitudinal pumping amplification crystals (C 1 , C 2 ); reflective optical elements (M′ 1 , M′ 2 , . . . , M′ 17 ) suitable for reflecting the optical beam so that the optical beam makes a total number of N sequential passes through the amplification crystals (C 1 , C 2 ), wherein N is an integer and N&gt;4. The reflective optical elements (M′ 1 , M′ 2 , . . . , M′ 17 ) are placed in a configuration suitable for alternatively interleaving the sequential optical beam passes through the 1 st  crystal (C 1 ) and through the 2 nd  crystal (C 2 ). A solid-state laser including the amplification system, and a method for amplifying a pulsed optical beam in a two-crystal multi-pass non-regenerative amplification system are also disclosed.

FIELD OF THE INVENTION

The present invention concerns a high power optical amplification systemand method. More particularly, the invention concerns an opticalamplifier based on solid-state gain medium, such as doped glasses orcrystals and a method for increasing output energy without transverselasing.

RELATED ART

There are mainly two types of solid-state amplification systems for highenergy, ultra short pulses: regenerative amplifiers and multipassamplifiers. A regenerative amplifier comprises an optical system forminga resonant cavity that includes the gain medium. In a regenerativeamplifier, the number N of passes of the optical beam through the gainmedium is very important (N>10). In contrast, a multipass amplifiergenerally comprises a thin, high gain medium and an optical system thatallows only a limited number of passes (N=2−8) of the optical beam to beamplified through the gain medium. The present invention concerns amultipass non-regenerative amplification system and method.

Multipass amplifiers are the core of solid-state laser systems. Suchlasers are required with increasing energy, power and shorter pulseduration. In particular, there is a need for high power lasers, in thePetawatt range. There is also a need for ultrashort pulse lasers, withhigh repetition rate and energy level around few tens of Joules perpulse.

The scaling of high energy laser for the production of very highintensity pulses requires large aperture solid-state gain medium. Forexample, a Ti-Sapphire laser uses one or several Titanium doped sapphirecrystals. The largest Ti:sapphire crystals available are cylinders ordisks of 10 cm diameter and a few centimeters length (1-5 cm).

Briefly, the optical amplification process is based on spontaneousemission of a gain medium when the amplifying medium is pumpedoptically. An optical amplifier generally comprises a non linear crystalthat is pumped at a pump wavelength λ_(P) different from the emissionwavelength λ_(e). The optical pumping is generally longitudinal, alongthe crystal cylinder axis and in the same direction as the amplifiedbeam propagation (propagative pumping) and/or in the opposite direction(contra-propagative pumping).

There are several solutions for increasing the output beam energy:increasing the pump power, increasing the gain-medium surface exposed tothe pump beam, and/or the gain-medium size.

Larger Ti:Sa crystals are used for increasing the pump absorption andthe overall gain of the amplifier.

Bonlie et al. (Production of >10²¹ W/cm² from a large apertureTi:sapphire laser system, Appl. Phys. B 70, 2000, S155-S160) describe alaser system using two Ti: sapphire amplifiers in “V” configurations,wherein the pulsed beam double-passes the first Ti:sapphire amplifier,and then double-passes the 2^(nd) Ti:sapphire amplifier. The twoamplifiers thus amplify the pulse sequentially, the pulse amplified bythe first amplifier being injected into the second amplifier.

However, Bonlie et al. disclose also that adverse effects of transverselasing occur as crystal diameter and pump power increase. Transverselasing is due to the formation of “laser cavities” inside the crystaland induced by the pumping beam (3, 4), as represented schematicallyFIG. 1. Total internal reflections (R=1) of the pumping beam on thecrystal edges can create parasitic transverse lasing (5), that decreasethe output beam energy.

For a cylindrical crystal 1, cavities are created in a plane transverseto the optical axis (2). In order to avoid multiple internalreflections, the input and output plane faces of crystal (1) are coatedwith an anti-reflection coating. However, pumping beams with an angle ofincidence above 36 degrees on the anti-reflection coated faces aretotally reflected. The optical losses (diffusion and absorption) dependon the index of reflection of the crystal and of the outside medium.Transverse lasing occurs depending on the product of the optical lossesby the volume gain.

Transverse gain, G^(t)(0), at the crystal surface for a cavity asrepresented in FIG. 1 is given by the following formula:

$\begin{matrix}{{G_{0}^{t}(0)} = {\exp\left( {\frac{{- \sigma_{e}}J_{0}\mu_{c}{\Phi\lambda}_{p}{\ln\left( {1 - A^{\lambda p}} \right)}}{h\; c\; l}\left\lbrack {2 - A^{\lambda p}} \right\rbrack} \right)}} & (1)\end{matrix}$

Where λ_(p) is the pump wavelength, A^(λp) is the crystal absorption atpump wavelength, σ_(e) is the amplification cross section at emissionwavelength λ_(e), J₀ pump fluence on a crystal face, φ the pump beamdiameter and μ_(c) the coupling efficiency, that defines non radiativelosses (depending on the crystal temperature). h is the planck constant(6.6 10⁻³⁴ J·s), c is the speed of light in vacuum (3 10⁸ m/s), and/thecrystal length.

We derive from equation (1) that, for a constant pump fluence,transverse gain G^(t) increases exponentially with the pump beamdiameter φ, whereas the extracted energy only increases quadratically.

The problem of transverse lasing thus becomes a major concern as higherenergy output beams are required.

Bonlie et al. disclose the use of an edge cladding around theTi:sapphire crystal, said cladding comprising a doped polymer with anabsorber for reducing transverse lasing. However, the aging of polymersand absorbing materials when exposed to high repetition laser pulses isunknown.

Besides, the use of increasingly larger Ti:sapphire single crystal(above 10 cm diameter) generates several issues. The qualityrequirements, availability and cost of such large size single crystalturn into reliability issues for the high energy laser chain.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide high power amplificationsystem and method with limited transverse lasing. Another goal of theinvention is to provide a stable, reliable and cost-effectiveamplification system and method.

More precisely, the invention concerns a high power solid-statenon-regenerative optical amplification system for amplifying a pulsedoptical beam, said amplification system comprising:

-   -   a first optical amplification crystal (C₁) and a second optical        amplification crystal (C₂) for amplifying said optical beam;    -   optical pumping means for longitudinal pumping amplification        crystals (C₁, C₂);    -   reflective optical means suitable for reflecting the optical        beam so that the optical beam makes a total number of N        sequential passes through said amplification crystals (C₁, C₂),        wherein N is an integer and N≧3.

According to the invention, the reflective optical means are placed in aconfiguration suitable for alternatively interleaving the sequentialoptical beam passes through the 1^(st) crystal (C₁) and through the2^(nd) crystal (C₂).

According to an embodiment of the invention, the reflective opticalmeans are arranged so that the optical beam makes multiple passesthrough the two crystals (C₁, C₂) including sequentially:

-   -   at least a first pass through 1^(st) amplification crystal (C₁),    -   at least a first pass through 2^(nd) amplification crystal (C₂),    -   at least another pass through 1^(st) amplification crystal (C₁),        and    -   at least another pass through 2^(nd) amplification crystal (C₂).

According to a preferred embodiment of the invention, the reflectiveoptical means are placed in “V” configuration so that the optical beammakes sequentially:

-   -   a first pass through 1^(st) amplification crystal (C₁),    -   a double pass through 2^(nd) amplification crystal (C₂),    -   a double pass through 1^(st) amplification crystal (C₁), and    -   a double pass through 2^(nd) amplification crystal (C₂).

Various embodiments the invention also concern the following features,that can be considered alone or according to all possible technicalcombinations and each bring specific advantages:

-   -   the total number N of passes through said amplification crystals        (C₁, C₂) is lower than 10;    -   said amplification crystals (C₁, C₂) are Titanium doped sapphire        crystals or Nd:glass;    -   said amplification crystals (C₁, C₂) have the same diameter Φ        and the to same thickness L;    -   said amplification crystals (C₁, C₂) have different sizes;    -   the maximum transverse gain G_(t) in the amplification crystals        (C₁, C₂) is lower than 50.

The invention also concerns a solid-state laser comprising anamplification system according to the invention.

In particular, the invention concerns a Petawatt laser comprising anamplification system according to the invention.

The invention also concerns a method for amplifying a pulsed opticalbeam in a two-crystals non-regenerative amplification system accordingto the invention and comprising the following steps:

-   -   longitudinally pumping two optical amplification crystals (C₁,        C₂);    -   injecting said optical beam said 1^(st) amplification crystal        (C₁);    -   reflecting said optical beam for multiple sequential passes        through the two optical amplification crystals (C₁, C₂), wherein        the multiple pass step includes alternatively interleaving the        optical beam passes through the 1^(st) crystal (C₁) and through        the 2^(nd) crystal (C₂) by means of the optical reflective        system.

According to a preferred embodiment of the method of the invention, themultiple pass step comprises the following steps:

-   -   at least a first pass through 1^(st) amplification crystal (C₁),    -   at least a first pass through 2^(nd) amplification crystal (C₂),    -   at least another pass through 1^(st) amplification crystal (C₁),        and    -   at least another pass through 2^(nd) amplification crystal (C₂).

According to a preferred method, the amplification process comprises thefollowing steps:

-   -   a first pass through 1^(st) amplification crystal (C₁),    -   a double pass through 2^(nd) amplification crystal (C₂),    -   a double pass through 1^(st) amplification crystal (C₁), and    -   a double pass through 2^(nd) amplification crystal (C₂).

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The following description is given as an example of the invention butcan have various embodiments that will be better understood whenreferring to the following figures:

FIG. 1 represents schematically a cross-section view of a Ti:sapphirecrystal and of transverse lasing induced by pump beams in the crystal;

FIG. 2 represents schematically a prior art multipass amplificationsystem comprising two amplification crystals, each in a bow-tieconfiguration;

FIG. 3 represents an example of energy amplified in a multipass systemas represented in FIG. 2 as a function of time, and for the successivepasses though the 1^(st) and 2^(nd) crystal;

FIG. 4 represents schematically a first embodiment of a multipassamplifier according to the invention;

FIG. 5 represents a simulation of energy build-up as a function of timeand as a function of interleaved passes through the 1^(st) and 2^(nd)crystals;

FIG. 6 represents schematically another embodiment of the multipassamplifier according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-section view of a large aperture amplification crystal(1) with longitudinal propagative and contra-propagative pump beams (3,4). The crystal (1) is generally a straight cylinder with disk shapedfaces of diameter Φ and length L and with an optical axis (2). Theamplification crystal (1) is pumped longitudinally by one or twolongitudinal pump beams (3, 4) for pumping the crystal through the twoflat faces. Fluorescence beam can propagate inside the crystal and bereflected on the flat faces and/or the outer surface. Losses can occurat the interfaces due to diffusion for example. However, for Ti:Sacrystal (refractive index n=1.76), if the angle of incidence of the beamis above 36 degrees, the beam is totally reflected (reflectioncoefficient R=1) and transverse lasing (5) can occur inside crystal (1).

For reference and future comparison with a double crystal multipasssystem, and with an embodiment of the invention, a prior art singlecrystal amplification system has the following operating parameters. Themaximum energy pump is set at 160 J and the maximum operating fluencesat 1 J·cm⁻² for a crystal having a diameter Φ of 15 cm, a length L of 3cm and 90% absorption at λ_(P)=532 nm. It is necessary to makes N=6passes through this single crystal for reaching saturation andamplification of the infrared (800 nm) pulse. In such conditions, theamplifier parameters are as follows:

-   -   Fluence at λ_(p)=532 nm:0.95 J·cm⁻²    -   Fluence at 800 nm:0.7 J·cm⁻²    -   Output energy after 6 passes:67 J    -   Pump beam diameter at λ_(p) (532 nm):12 cm    -   Optical beam diameter at 800 nm:11.5 cm    -   Maximum transverse gain G^(t):400

The transverse gain for a single crystal amplifier is extremely large.The above example confirms that transverse lasing becomes a major issuewith crystal dimensions (Φ and L) and with the beam fluence.

FIG. 2 represents a prior art amplification system comprising twoamplifiers (A₁, A₂) each schematically surrounded by a dashed line.Optical pumping means are not represented on FIG. 2. Pumping beams areassumed to be conventional longitudinal propagative andcontra-propagative beams. Each amplifier (A₁, A₂) comprises anamplification crystal (C₁, C₂) and an optical system for multipassamplification through each crystal.

Considering the 1^(st) amplifier A₁, the optical system is a reflectiveoptical system comprising mirrors M₁-M₇. The mirrors are arranged in awell-known bow-tie configuration, for enabling the optical beam to passN_(i) times through amplification crystal 1. The input beam S_(in) isdirected by mirror M₁ through the first crystal (C₁). S₁ represents theoptical beam S_(in) amplified after the first pass through first crystal(C₁). S₁ propagates and is reflected successively by mirrors M₂ and M₃towards the first crystal (C₁) for a second pass. After passing through(C₁), the beam S₁ is amplified into S₂. S₂ is reflected by mirrors M₄and M₅ and directed for a third pass through first crystal (C₁). Afterthe third pass, amplified beam S₃ is reflected by mirrors M₆ and M₇ anddirected for a fourth pass through first crystal (C₁) and amplified intoS₄ beam. In the example of FIG. 2, the optical beam is amplifiedsuccessively by N₁=4 passes through crystal (C₁). Multiple passes enableto reach saturation inside crystal (C₁) and thus maximum amplification.

An intermediate reflective optical system (mirrors M₈-M₁₀) transfers theoptical beam amplified by the 1^(st) amplifier A₁ and injects it intothe 2^(nd) amplifier (A₂). S₄ beam exits out of Amplifier 1 and isdirected by the mirrors M₈, M₉ and M₁₀ towards a second amplificationstage.

Similarly to the first amplifier A₁, the second amplifier A₂ comprises asecond amplification crystal (C₂) and a reflective optical system(mirrors M₁₁-M₁₇), for passing the optical beam N₂ times through crystal(C₂). S₄ beam is reflected by mirror M₁₁ and directed for a first passthrough second crystal C₂. After passing through (C₂), the beam S₄ isamplified into S₅. Amplified beam S₅ is reflected by mirrors M₁₂ and M₁₃and directed for a second pass through crystal (C₂). After the secondpass through C₂, amplified beam S₆ is reflected by mirrors M₁₄ and M₁₅and directed for a third pass through second crystal (C₂) and amplifiedinto S₇ beam. Beam S₇ is reflected by mirrors M₁₆ and M₁₇ and directedfor a fourth pass through second crystal (C₂) and amplified into S₈beam. Beam S₈ is thus amplified successively four times through firstcrystal C₁ and then four times through second crystal C₂.

As a reference for future comparison with an embodiment of theinvention, a prior art system as represented on FIG. 2 comprises twoTitanium:sapphire crystals of same length (3 cm) and absorption 90% at532 nm.

The 1^(st) amplifier (A1) parameters are as follows:

-   -   Crystal Diameter Φ₁:7.5 cm    -   Pump beam diameter at λ_(p) (532 nm):6 cm    -   Optical beam diameter at 800 nm:5.5 cm    -   Pump energy:40 J/Fluence at λ_(p) (532 nm):0.94 J·cm⁻²    -   Input energy at 800 nm:5 J/Maximum fluence:0.9 J·cm⁻²    -   Output energy after 4 passes:20 J    -   Maximum transverse gain G^(t):100

The 2^(nd) amplifier (A₂) parameters are as follows:

-   -   Crystal Diameter Φ₂:12.5 cm    -   Pump beam diameter at λ_(p) (532 nm):10 cm    -   Optical beam diameter at 800 nm:9.5 cm    -   Pump energy (532 nm):120 J/Fluence at λ_(p):1 J·cm⁻²    -   Input energy at 800 nm:20 J/Maximum fluence:1 J·cm⁻²    -   Output energy after 4 passes:67.5 J    -   Maximum transverse gain G^(t):200

FIG. 3 represents the progressive amplification of the optical beam in aprior art two crystals amplification system, with above parameters,wherein the optical beam makes 4 passes inside each amplificationcrystal (total N=8). The lower curve corresponds to A₁ amplification,and the upper curve to A₂ amplification. The input energy in A₁ is 5 Jat 800 nm. The output energy of A₁ is 20 J after 4 passes. The opticalbeam amplified by A₁ is the injected into A2 and amplified again. Theoutput energy after 4 passes through A₂ is 67.5 J. In the examplerepresented FIG. 3, we observe a progressive saturation of the energyfor each crystal, and the 4^(th) path in each crystal appearsunnecessary.

In summary, prior art multipass amplification system, as illustrated inFIGS. 2-3 comprise two crystals for serial amplification of the opticalbeam, up to the maximum gain corresponding to the sum of the gains ofthe two crystals (C₁, C₂).

FIG. 4 represents schematically a first embodiment of a multipassamplifier according to the invention. The amplification system (100)comprises two amplification crystals (C₁, C₂). Optical pumping means arenot represented on FIG. 4. Pumping beams are assumed to be conventionallongitudinal propagative and contra-propagative beams. The amplificationsystem also comprises an optical system (M′₁-M′₁₃) for multipassamplification through the two crystals (C₁, C₂). However, in contrastwith prior art multiple crystals amplification system, the optical beamdoes not follow a serial amplification through the different crystals,with a first amplification in a first crystal and then sequentially asecond amplification in the 2^(nd) crystal.

As evidenced on FIG. 4, the input optical beam S′_(i) makes a first passthrough the 1^(st) amplification crystal (C₁), and forms an amplifiedbeam S′₁. Mirrors M′₂-M′₃ inject the S′₁ beam into the secondamplification crystal (C₂). After a first pass through 2^(nd)amplification crystal (C₂) the amplified beam is labelled S′₂. In theembodiment of FIG. 4, the amplified beam S′₂ is reflected by mirrorsM′₄-M′₅ for passing again through the 2^(nd) crystal (C₂) and formsamplified beam S′₃. Then, the amplified beam S′₃ is injected usingmirrors M′₆-M′₇ into the first crystal (C₁) for another pass through the1^(st) crystal (C₁). Mirrors M′₈-M′₉ inject amplified beam S′₄ into the1^(st) crystal for a 3^(rd) pass through this 1^(st) crystal (C₁).Mirrors M′₁₀-M′₁₁ inject amplified beam S′₅ into the 2^(nd) crystal (C₂)for a 3^(rd) pass, thus forming amplified beam S′₆. Mirrors M′₁₂-M′₁₃fold amplified beam S′₆ and inject it for a fourth pass through crystal(C₂). Mirror M′₁₄ extracts the amplified S′₇ beam out of amplificationsystem (100).

In summary, the optical beam makes a total of N=7 passes through theamplification crystals, including three passes through crystal (C₁) andfour passes through crystal (C₂). In contrast to prior art multi-crystalamplification systems, the passes through the different crystals (C₁,C₂) are interleaved. More precisely, the sequential passes through thefirst crystal C₁ and through the second crystal C₂ are alternativelyinterleaved. In the above example, after the 1^(st) pass through the1^(st) crystal (C₁), following passes are double-passes alternativelythough the 2^(nd) and 1^(st) crystal.

Alternatively, the 1^(st) pass in C₁ can be a double pass.

In an example, the length of each crystal (C₁, C₂) is 3 cm and theirdiameter 12.5 cm. The pump beam wavelength is 532 nm. The crystalabsorption at 532 nm is 90%. The pump beam diameter is 10 cm, and thediameter of the optical beam (to be amplified) is 9.5 cm. The overallpump energy is 80 J for each crystal, and the pump fluence at 532 nm is0.7 J·cm⁻². The input optical beam energy is 5 J at 800 nm, and themaximum fluence is 0.92 J·cm⁻². The output energy after 8 passes is 68.3J. The maximum transverse gain is G^(t)=40 in each crystal (C₁, C₂).

-   -   Crystal Diameter Φ₁=Φ₂:12.5 cm    -   Pump beam diameter at λ_(p) (532 nm):10 cm    -   Optical beam diameter at 800 nm:9.5 cm    -   Pump energy (532 nm):80 J/Fluence at λ_(p):0.7 J·cm⁻²    -   Input energy at 800 nm:5 J/Maximum fluence:0.92 J·cm⁻²    -   Output energy after 8 passes (total):68.3 J    -   Maximum transverse gain G^(t):40

FIG. 5 represents the progressive amplification of the optical beam inan example corresponding to the configuration of FIG. 4 using the aboveoperating parameters.

We observe a regular amplification, almost linear by steps, during theinterleaved passes through the 1^(st) and 2^(nd) amplification crystalsC₁ and C₂.

The energy at the output of the amplification system represented in FIG.5 is 68.7 J for 5 J input energy, which corresponds approximately to thesame energy levels as observed for the system presented in FIGS. 2-3.The overall gain of the prior art system and of the embodiment of theinvention are thus similar.

However, as compared to prior art system, the transverse gain insideboth amplification crystals of the preferred embodiment of the inventionis much lower: G^(t)=40 instead of 200.

In addition, the pump density is also lower on both crystals (0.7J·cm⁻², instead of 1 J·cm⁻²), resulting in a higher transverse lasingthreshold, better extraction, and in an improved crystal protectionagainst damage.

FIG. 6 represents schematically another embodiment of the invention. Inthe embodiment of FIG. 6, the beam makes single passes alternativelythrough the first crystal (C1) and through the second crystal (C2).

As evidenced on FIG. 6, the input optical beam S′_(i) is reflected bymirror M′₁, makes a first pass through the 1^(st) amplification crystal(C₁), and forms an amplified beam S′₁. Mirrors M′₂-M′₃ inject the S′₁beam into the second amplification crystal (C₂). After a first passthrough 2^(nd) amplification crystal (C₂) the amplified beam is labelledS′₂. In the embodiment of FIG. 6, the amplified beam S′₂ is reflected bymirrors M′₄-M′₅ for passing again through the 1^(st) crystal (C₁) andforms amplified beam S′₃. Then, the amplified beam S′₃ is injected usingmirrors M′₆-M′₇ into the second crystal (C₂) for another pass throughthe 2^(nd) crystal (C₂). Mirrors M′₈-M′₉ inject amplified beam S′₄ intothe 1^(st) crystal for a 3^(rd) pass through this 1^(st) crystal (C₁).Mirrors M′₁₀-M′₁₁ inject amplified beam S′₅ into the 2^(nd) crystal (C₂)for a 3^(rd) pass, thus forming amplified beam S′₆. Mirrors M′₁₂-M′₁₃inject amplified beam S′₆ into the 1^(st) crystal for a 4^(th) passthrough this 1^(st) crystal (C₁). Mirrors M′₁₄-M′₁₅ inject amplifiedbeam S′₇ into the 2^(nd) crystal (C₂) for a 4^(th) pass, thus formingamplified beam S′₈. Mirrors M′₁₆-M′₁₇ inject amplified beam S′₈ into the1^(st) crystal for a 5^(th) pass through first crystal (C₁), thusforming amplified beam S′₉. Mirror M′₁₈ extracts the amplified S′₉ beamout of amplification system (100).

In summary, in the embodiment of FIG. 6, the optical beam makes a totalof N=9 interleaved passes through the amplification crystals, includingfive passes through crystal (C₁) and four passes through crystal (C₂).

In contrast to prior art multi-crystal amplification systems, the passesthrough the different crystals (C₁, C₂) are interleaved. More precisely,the sequential passes are alternatively interleaved through thedifferent crystals.

In the embodiment of FIG. 4, after the 1^(st) pass through the 1^(st)crystal (C₁), following passes are double-passes alternatively thoughthe 2^(nd) and 1^(st) crystal. Double passes through a crystal areperformed in opposite directions along the crystal optical axis.

In the embodiment of FIG. 6, after the 1^(st) pass through the 1^(st)crystal (C₁), following passes are single passes alternatively thoughthe 2^(nd) and 1^(st) crystal. The beam passes through a crystal are allin the same direction.

According to various embodiments of the invention, each amplificationcrystal (C₁, C₂, . . . , C_(M)) can be temperature controlled. Forexample, the temperature of each crystal (C_(i))_(i=1 . . . M) can becontrolled independently in order to control the gain of eachamplification medium.

Different crystals (C₁, C₂, . . . , C_(M)) having different dopinglevels can also be used in order to control the gain of eachamplification medium.

Another advantage of the system and method of the invention is that theuse of multiple (minimum two) amplification crystals provides smoothingof to crystal defects.

The invention provides an improved system stability (large number ofpump beams). The alternatively interleaved pass configuration allows tobalance saturation among the two (or more) crystals. In the prior artserial configuration, most of the amplification process occurs duringthe first two passes through each crystal. In contrast, the interleavedpass configuration of the invention produces a significant amplificationat each pass. In prior art multi-crystal configuration, the secondcrystal is exposed to very high infrared fluence, that can bedestructive. The interleaved configuration is less stringent relativelyto pumping and guarantees a higher long terme stability (laser pumpdrift is less critical).

The multipass amplification method according to the inventioninterleaves amplification between different amplification crystals. Thismethod enables progressive saturation of the different amplificationmedium. The balanced saturation among the two amplification crystalsprovides long term stability of the system.

The system and method of the invention apply to a high power solid-statelaser, and in particular to a Petawatt laser system.

In a preferred embodiment, the amplification system of the inventioncomprises two amplification crystals.

However, the amplification system can be scaled for higher amplificationgain, using more than two amplification crystals, without increasing theamplification crystal size. The pump fluence remains also limited on allamplification crystals.

The invention applies to high power laser, and in particular lasershaving either low repetition rate and high energy, or high repetitionrate and low energy.

1. High power solid-state non-regenerative optical amplification system(100) for amplifying a pulsed optical beam, the amplification systemcomprising: a first optical amplification crystal (C₁) and a secondoptical amplification crystal (C₂) for amplifying said optical beam;optical pumping means for longitudinal pumping amplification crystals(C₁, C₂); reflective optical means (M′₁, M′₂, . . . , M′₁₇) suitable forreflecting the optical beam so that the optical beam makes a totalnumber of N sequential passes through said amplification crystals (C₁,C₂), wherein N is an integer and N≧3; characterized in that: thereflective optical means (M′₁, M′₂, . . . , M′₁₇) are placed in aconfiguration suitable for alternatively interleaving the sequentialoptical beam passes through the 1^(st) crystal (C₁) and through the2^(nd) crystal (C₂).
 2. High power amplification system according toclaim 1 wherein the reflective optical means (M′₁, M′₂, . . . , M′₁₇)are arranged so that the optical beam makes multiple passes through thetwo crystals (C₁, C₂) including sequentially: at least a first passthrough 1^(st) amplification crystal (C₁), at least a first pass through2^(nd) amplification crystal (C₂), at least another pass through 1^(st)amplification crystal (C₁), and at least another pass through 2^(nd)amplification crystal (C₂).
 3. High power amplification system accordingto claim 2 characterized in that the reflective optical means (M′₁, M′₂,. . . , M′₁₇) are placed in “V” configuration so that the optical beammakes sequentially: a first pass through 1^(st) amplification crystal(C₁), a double pass through 2^(nd) amplification crystal (C₂), a doublepass through 1^(st) amplification crystal (C₁), and a double passthrough 2^(nd) amplification crystal (C₂).
 4. High power amplificationsystem according to claim 1 characterized in that the total number N ofpasses through said amplification crystals (C₁, C₂) is lower than
 10. 5.High power amplification system according to claim 1 characterized inthat said amplification crystals (C₁, C₂) are chosen among Titaniumdoped sapphire crystals and Nd:Glass.
 6. High power amplification systemaccording to claim 1 characterized in that said amplification crystals(C₁, C₂) have the same diameter Φ and the same thickness L.
 7. Highpower amplification system according to claim 1 characterized in thatsaid amplification crystals (C₁, C₂) have different sizes.
 8. High poweramplification system according to claim 1 wherein the maximum transversegain G_(t) in the amplification crystals (C₁, C₂) is lower than
 50. 9.Solid-state laser comprising an amplification system according toclaim
 1. 10. Petawatt laser comprising an amplification system accordingto claim
 1. 11. Method for amplifying a pulsed optical beam in atwo-crystals non-regenerative amplification system according to claim 1comprising the steps of: longitudinally pumping two opticalamplification crystals (C₁, C₂); injecting said optical beam into said1^(st) amplification crystal (C₁); reflecting said optical beam formultiple sequential passes through the two optical amplificationcrystals (C₁, C₂) wherein the multiple pass step includes alternativelyinterleaving the optical beam passes through the 1^(st) crystal (C₁) andthrough the 2^(nd) crystal (C₂) by means of the optical reflectivesystem (M′₁, M′₂, . . . , M′₁₇).
 12. Method for amplifying a pulsedoptical beam according to claim 11 wherein the optical beam is reflectedso that it makes: at least a first pass through 1^(st) amplificationcrystal (C₁), at least a first pass through 2^(nd) amplification crystal(C₂), at least another pass through 1^(st) amplification crystal (C₁),and at least another pass through 2^(nd) amplification crystal (C₂). 13.Method according to claim 12 wherein the amplification process comprisesthe following steps: a first pass through 1^(st) amplification crystal(C₁), a double pass through 2^(nd) amplification crystal (C₂), a doublepass through 1^(st) amplification crystal (C₁), and a double passthrough 2^(nd) amplification crystal (C₂).